1. Field of the Invention
The present invention relates to a dynamic random access memory (dynamic RAM) formed by a MOS process.
2. Description of the Prior Art
A dynamic RAM generally employs a memory cell comprising a single transistor and a single capacitor. In this case, the smaller the ratio of capacitance of a bit line to capacitance of a capacitor in a memory cell is, the larger amount of change in potential on a bit line at the time of read-out is, and the larger input potential difference for a sense amplifier is, so that read-out operation is ensured. However, as density of a memory is largerly increased and integration increases, the size of a memory cell becomes smaller, so that capacitance of a capacitor is reduced. On the other hand, since the number of memory cells connected to a single bit line increases, a bit line becomes longer, so that capacitance of a bit line tends to increase. As a result, the ratio of capacitance of a bit line to capacitance of a capacitor increases, so that read-out operation may not be ensured.
To solve this problem, a single bit line is divided into a plurality of blocks so that the ratio of capacitance of a capacitor and capacitance of a bit line may be reduced. Two examples related to such a trial are described in the following.
FIG. 3 is a diagram showing a part of a structure of a conventional dynamic RAM, which is disclosed in an article of Digest of Technical Papers, ISSCC 84, pp. 278-279. In FIG. 3, a so-called shared sense amplifier structure is shown wherein a pair of bit lines are divided into two parts BL1 and BL2 and and a sense amplifier is shared with each pair of divided bit lines. In the above described document, a transistor in a memory cell comprises a P channel transistor, a sense amplifier comprises a P channel transistor, and a restore circuit comprises an N channel transistor. For simplicity, the conductivity type of these transistors is inverted in FIG. 3.
In FIG. 3, a pair of bit lines constituting a folded bit line are divided into divided bit lines BL1, BLN and BL2 and divided bit lines BL1, BLN and BL2, respectively. A sense amplifier SA is connected to the divided bit lines BLN and BLN, a restore circuit RE1 is connected to the divided bit lines BL1 and BL1, and a restore circuit RE2 is connected to the divided bit lines BL2 and BL2. The sense amplifier SA comprises N channel transistors QN1 and QN2 as described above, and the restore circuits RE1 and RE2 comprise P channel transistors QP1 and QP2 and QP3 and QP4, respectively.
The transistors QN1 and QN2 have sources connected to a common sense amplifier driver transistor QN5. The transistor QN5 has a gate receiving a sense amplifier activating signal SN. The transistors QP1 and QP2 have sources connected to a common restore circuit driver transistor QP5, and the transistors QP3 and QP4 have sources connected to a common restore circuit driver transistor QP6. The transistors QP5 and QP6 have gates receiving restore circuit activating signals SP1 and SP2, respectively.
The divided bit lines BL1 and BLN and BLN and BL2 are connected to each other through transfer gate transistors QT1 and QT3, respectively, and the divided bit line BL1 and BLN and BLN and BL2 are connected to each other through transfer transistors QT2 and QT4, respectively. The transfer gate transistors QT1 and QT2 have gates receiving a transfer signal T1, the transfer gate transistors QT3 and QT4 have gates receiving a transfer signal T2.
The divided bit lines BL1 and BL1 are connected to bus lines BU and BU through column gate transistors QY1 and QY2. The column gate transistors QY1 and QY2 have gates receiving a column selecting signal Y.
Although a plurality of memory cells are connected to each of the divided bit lines, depending on memory capacity, only a memory cell MC1 connected to the divided bit line BL2 is typically shown herein. The memory cell MC1 comprises a capacitor CS and a transistor QS. The transistor QS has a gate being a part of a word line WL1. In addition, the capacitor CS has one electrode connected to a memory cell plate potential V.sub.SG.
Referring now to FIG. 4 which is a waveform diagram showing operation, operation of the circuit shown in FIG. 3, is described when the capacitor CS in the memory cell MC1 connected to the divided bit line BL2 is not charged, that is, when information "0" is stored in the memory cell MC1.
At the time t.sub.0, the transfer signal T1 becomes an "L" level. Accordingly, the divided bit lines BLN and BL1 and BLN and BL1 are isolated, respectively. Before the time t.sub.0, the divided bit lines BL1, BL1, BL2, BL2, BLN and BLN are precharged at an intermediate potential V.sub.CC -V.sub.SS)/2. At the time t.sub.1, the selected word line WL1 becomes an "H" level. Accordingly, the transistor QS is turned on. As a result, the potential on the divided bit line BL2 slightly lowers, so that potential difference occurs between the divided bit lines BL2 and BL2. At the time t.sub.2, the sense amplifier activating signal SN becomes an "H" level. As a result, potential difference is increased between the divided bit lines BL2 and BL2. More specifically, the potential on the divided bit line BL2 is held near the intermediate potential, while the divided bit line BL2 is discharged through the transfer gate transistor QT3 and the sense amplifier SA, so that the potential thereon becomes near a ground potential V.sub.SS. At the time t.sub.3, the restore circuit activating signal SP2 becomes an "L" level. Accordingly, the potential on the divided bit line BL2 is pulled up near a power supply potential V.sub.CC. As a result, potential difference is further increased between the divided bit lines BL2 and BL2. At the time t.sub.4, the transfer signal T1 becomes again an "H" level. Thus, the potentials on the divided bit lines BLN and BLN are transferred to the divided bit lines BL1 and BL1, respectively. As a result, the divided bit line BL1 is discharged, so that the potential thereon becomes near the ground potential V.sub.SS, while the potential on the divided bit line BL1 is pulled up. At the time t.sub.5, the restore circuit activating signal SP1 becomes an "L" level. As a result, the potential on the divided bit line BL1 is pulled up near the power supply potential V.sub.CC. At the time t.sub.6, the column selecting signal Y becomes an "H" level. Thus, the potentials on the divided bit lines BL1 and BL1 are transferred to the bus lines BU and BU, so that information "0" stored in the memory cell MC1 is read out.
As described in the foregoing, information stored in the capacitor CS in the memory cell MC1 is first read out to the divided bit line BL2, so that potential difference between the divided bit lines BL2 and BL2 is amplified by the sense amplifier SA. The divided bit line BL2 is discharged at the sense amplifier SA through the transfer gate transistor QT3. The dynamic RAM with a folded bit line structure is generally formed of low resistance materials such as aluminum or refractory metal silicide. As a result, resistance of a bit line can be reduced, so that discharge of charges on the bit line can be accelerated.
However, in the dynamic RAM with a shared sense amplifier structure as described above, a transfer gate transistor is provided between a divided bit line connected to a memory cell and a sense amplifier, so that a bit line is not formed of low resistance materials in this transistor portion. In addition, as shown in FIG. 3, since the transfer gate transistor must be provided for each pitch between bit lines, the transistor width can be made almost the same as or at most twice the pitch between bit lines. In consideration of the pitch between bit lines, the pitch between bit lines is, for example, about 3 .mu.m in a 1 Mega-bit dynamic RAM. Therefore, the transistor width of the transfer gate transistor is limited to less than several .mu.m. Accordingly, conductance of the transfer gate transistor is reduced, so that discharge of charges on the divided bit line is delayed when the sense amplifier operates.
Furthermore, since a source and a drain of a transfer gate transistor are formed of a diffusion layer provided in a substrate or a well, noise is transferred to a bit line through the substrate or the well, so that the sense amplifier erroneously operates.
FIG. 5 illustrates a circuit which is disclosed in the Japanese Laying-Open Gazette No. 101093/1984, as another conventional example of interest to the present invention. The circuit shown in FIG. 5 comprises only an N channel transistor, and a bit line is divided into three parts. The divided bit lines BL4 and BL4 are connected to an active pull-up circuit AP and a bit line precharge circuit BC. The transfer gate transistors QT1, QT2, QT3 and QT4 are connected between the divided bit lines, and the column gate transistors QY1 and QY2 are connected between the divided bit line BL4 and the bus line BU and between the divided bit line BL4 and the bus line BU. A sense amplifier SA5 is connected to the divided bit lines BL5 and BL5, while a sense amplifier SA6 is connected to the divided bit lines BL6. Furthermore, although a plurality of memory cells are connected to each of the divided bit lines, respectively, only a memory cell MC1 connected to the divided bit line BL5 out of memory cells connected to each of the divided bit lines is shown herein. The memory cell MC1 comprises the capacitor CS and the transistor QS. The transistor QS has a gate being a part of the word line WL1. The capacitor CS has on electrode connected to the memory cell plate potential V.sub.SG.
Referring now to FIG. 6 showing waveforms of operation, operation of the circuit shown in FIG. 5 is described when the capacitor CS in the memory cell MC1 is not charged, that is, when information "0" is stored in the memory cell MC1.
Before the time t.sub.0, both a transfer signal BSC and a reset signal RST are at an "H" level, and all the transfer gate transistors QT1, QT2, QT3 and QT4 are turned on. Thus, the divided bit lines BL4, BL5 and BL6 are connected to each other, and the divided bit lines BL4, BL5 and BL6 are connected to each other. In addition, since the reset signal RST becomes an "H" level, a bit line precharge circuit BC operates. As a result, each of the divided bit lines is precharged, so that the potential thereon becomes the intermediate potential (V.sub.CC -V.sub.SS)/2.
At the time t.sub.0, both the transfer signal BSC and the reset signal RST become an "L" level. At the time t.sub.1, the potential on the selected word line WL1 becomes an "H" level. As a result, the potential on the divided bit line BL5 slightly lowers, so that potential difference occurs between the divided bit lines BL5 and BL5. At the time t.sub.2, a sense amplifier activating signal SN5 become an "H" level. Accordingly, the sense amplifier SA5 operates. As a result, so that potential difference is increased between the divided bit lines BL5 and BL5. At the time t.sub.3, the transfer signal BSC becomes an "H" level. Accordingly, the transfer gate transistors QT1, QT2, QT3 and QT4 are turned on, so that the potential on the divided bit lines BL5 and BL5 are transferred to the divided bit lines BL4 and BL6 and BL4 and BL6. respectively. At the time t.sub.4, the sense amplifier activating signal SN6 becomes an "H" level. As a result, potential difference is increased between the divided bit lines BL6 and BL6 and thus, potential difference is increased between the divided bit lines BL4 and BL4 and between the divided bit lines BL5 and BL5. At the time t.sub.5, an active pull-up signal APE becomes an "H" level. Accordingly, an active pull-up circuit APE operates. As a result, the potential on the divided bit line BL4, BL5 and BL6 is pulled up near the power supply potential V.sub.CC. When the column selecting signal Y becomes an "H" level, the potentials on the divided bit lines BL4 and BL4 are transferred to the bus lines BU and BU, respectively, so that information is read out.
As described in the foregoing, in the circuit shown in FIG. 5, a sense amplifier is provided for each of the divided bit lines, while an active pull-up circuit is provided not for each of the divided bit lines but for an entire bit line. Therefore, since the potential on the entire bit line must be pulled up by an active pull-up circuit when the active pull-up circuit operates, an active pull-up circuit having large drive capacity is required. This increases the area occupied by the active pull-up circuit.
Additionally, in order to pull up the potential on each of the divided bit lines to the power supply potential V.sub.CC, the gate potential of the transfer gate transistor, that is, the transfer signal BSC must be boosted over the power supply potential V.sub.CC. However, as integration of a memory increases, a gate oxide film of the transistor tends to be thinner. For example, the gate oxide film in a 1 Mega-bit dynamic RAM is about 200 to 300.ANG. in thickness. Therefore, if the gate potential is boosted over the power supply potential, reliability of the gate oxide film is deteriorated.